The present invention relates to a heat treating method and device.
The fabrication process for fabricating a semiconductor wafer (hereinafter called "wafer") includes processing of heat treatments for film deposition, impurities diffusion, etc. As devices for conducting such heat treatments, recently vertical heat treating devices are more used in place of horizontal heat treating devices because the former takes the outside air into the processing chambers.
FIG. 6 shows a conventional vertical heat treating device. This vertical heat treating device includes a heat processing furnace having a reaction tube 1 of a double structure of a vertical inner tube 1a and a vertical outer tube 1b, and a heater 11 provided around the reaction tube 1. A wafer boat 12 with a number of sheets of wafers W supported in a shelves-like manner is loaded by means of a boat elevator into the heat processing furnace from below for a heat treatment. For suppressing intrusion of air, nitrogen gas (N.sub.2) is introduced into the heat processing furnace before a cap 14 on the lower end of the heat processing furnace is lowered when the heat treatment is over, i.e., before the wafer boat 12 is lowered. A pressure switch 16 for detecting an internal pressure of the heat processing furnace is set so as to be turned on when an internal pressure of the heat processing furnace becomes a little higher than the air pressure. When the pressure switch 16 is turned on, a control unit 17 supplies a drive instruction to a motor 18 of the boat elevator 13 to lower the wafer boat 12.
Here, the air pressure largely varies depending on pressure distributions, and an internal pressure of the heat processing furnace at the time when the set pressure switch is turned on is always constant. Accordingly a large pressure difference often takes place between the inside of the heat processing furnace and the outside thereof when the cap 14 is opened. In such cases, large turbulent air flows take place near the opening of the heat processing furnace when the cap 14 is opened and causes particles to float. These particles stay on the surfaces of the wafers W.
Recent devices are so integrated that their pattern widths are very micronized. Allowances for the deprint of the particles are more restricted. Deposit of the particles on the wafers when the wafers are unloaded out of the heat processing furnace is one factor of low yields of the devices. This is one problem.
When an outside air pressure is higher than an internal pressure of the heat processing furnace when the cap 14 is opened, the air intrudes into the heat processing furnace, and the surfaces of the wafers W contact with oxygen in a high-temperature ambient air. Accordingly there is a risk that natural oxide films will be formed on the surfaces of of the wafers W, especially those located below the wafer boat 12. Here, in a case where a next step is vulnerable to natural oxide films, the surfaces of the wafers W are rinsed. But the thus-formed oxide films have disuniform film thicknesses, and inhomogeneous, so it is difficult to evenly remove them by the rinse. For example, residual poor-quality oxide films in silicon nitride films of multi-layer insulating films which are increasingly used in accordance with higher integration of DRAMs degrades characteristics of the devices. This is also a factor of lower yields of the devices. This is also a problem.
In heat treatments using harmful substances, such as phosphorus diffusion or others, residual phosphorus-content gases after the treatments often cause environmental problems. Also particles often stay on the wafer boat for mounting the wafers, etc. As treatments are repeated with particles residing in the space of the opening of the reaction tube, a number of the particles increases, and the particles intrude into the reaction vessel whenever the wafer boat is loaded into and unloaded out of the reaction vessel. As heat treatments are repeated, the particles stay on the wafers. Accordingly treatment precision is lowered, and accordingly yields of the devices are lowered.
In a case that, as shown in FIG. 7, maintenance is conducted when a 10th, for example, treatment is over, preferably a particle number does not exceed a reference value as in A, but as in B sometimes the reference value is exceeded when a ninth treatment is over. A maintenance timing has been conventionally set based on treatment times. Accordingly even in a case that although a treatment time is few, a particle number have increased, treatments are still repeated. Even in the presence of a number of particles above a reference value in the reaction tube, treatments are conducted on the wafers with particles staying on. Accordingly the treatment precision is lowered, and yields of the devices are lowered.